Projection-type image display apparatus

ABSTRACT

A projection-type image display apparatus that enlarges and displays an image includes: a case, a laser light source, a cooling mechanism that includes a component that produces vibration during operation, a diffuser that diffuses light that is emitted from the laser light source, optics that shape light diffused by the diffuser to light having a uniform luminance distribution and rectangular cross-section, and an optical modulation element that modulates light shaped by the optics; wherein the component included in the cooling mechanism and the diffuser are connected by way of a vibration transmitting member.

TECHNICAL FIELD

The present invention relates to a projection-type image displayapparatus that uses a laser light source.

BACKGROUND ART

In recent years, laser light sources are receiving attention as onelight source of a projection-type image display apparatus of whichprojectors are representative. A laser light source has severaladvantages. First, laser light emitted from a laser light sourcefeatures superior directivity and therefore features high opticalutility efficiency. In addition, laser light is monochromatic andtherefore can broaden the color reproduction region. A laser lightsource also features low power consumption and long life.

FIG. 1 shows a schematic block diagram of a projector that uses a laserlight source. Projector 1 shown in FIG. 1 includes at least: laser lightsources 2 a-2 c corresponding to each of the primary color signalsR/G/B; collimator lenses 3 a-3 c, light tunnels 4 a-4 c, opticalmodulation elements (liquid crystal panels) 5 a-5 c, dichroic prism 6,and projection lens 7.

Polarization beam splitters (PBS) 8 a-8 c are arranged between each oflaser light sources 2 a-2 c and each of collimator lenses 3 a-3 c. Inaddition, incidence-side polarizing plates 9 a-9 c are arranged on thelight incidence side of each of liquid crystal panels 5 a-5 c.Emission-side polarizing plates 10 a-10 c are arranged on the lightemission sides of each of liquid crystal panels 5 a-5 c.

The operation of projector 1 shown in FIG. 1 is next summarized. Laserlight beams 12 a-12 c emitted from each of laser light sources 2 a-2 c,respectively, are converted to specific linearly polarized light bypolarization beam splitters (PBS) 8 a-8 c and pass through collimatorlenses 3 a-3 c, respectively. Laser light beams 12 a-12 c that havepassed through collimator lenses 3 a-3 c are directed into light tunnels4 a-4 c. The beam diameters of laser light beams 12 a-12 c that havepassed through collimator lenses 3 a-3 c are gradually enlarged untilbeing irradiated into light tunnels 4 a-4 c.

Light tunnels 4 a-4 c are hollow prisms. A reflective film is applied byvapor deposition to the inner wall surfaces of light tunnels 4 a-4 c.Laser light beams 12 a-12 c irradiated into light tunnels 4 a-4 c fromone opening of each of light tunnels 4 a-4 c, respectively, advancetoward the other opening while being repeatedly reflected inside lighttunnels 4 a-4 c. In the process of advancing inside light tunnels 4 a-4c, not only is the luminance distribution in the luminous fluxcross-sections of laser light beams 12 a-12 c equalized, but thesectional profiles are reshaped into a rectangular form.

Laser light beams 12 a-12 c that are emitted from each of light tunnels4 a-4 c are irradiated into corresponding liquid crystal panels 5 a-5 c,respectively. Laser light beams 12 a-12 c that have been irradiated intoliquid crystal panels 5 a-5 c undergo optical modulation according toimage signals. The light that has undergone optical modulation issynthesized by dichroic prism 6 and enlarged and projected onto screen11 by way of projection lens 7.

However, when coherent light such as laser light is irradiated onto arough surface (such as a screen) having unevenness that is greater thanthe wavelength of the light, a mottled light pattern referred to as a“speckle pattern” or “speckle” is produced. More specifically, light ofa single wavelength that is scattered at each point on a rough surfaceoverlaps irregularly at each point on the observed surface to produce acomplicated interference pattern.

Thus, when an image is projected onto a screen by a projector that usesa laser light source, the laser light is diffused on the screen surfaceand strong random noise (speckled noise) is produced. When this speckleis formed as an image on an observer's retina in this case, the speckleis perceived as unfocused mottled flickering, and this causes discomfortand fatigue for the observer. The observer further senses extremedegradation of the image quality.

In the field of projectors that employ laser light sources, variousmethods have been proposed for reducing the above-described specklenoise.

Typically, two approaches exist as methods for reducing speckle noise.One involves making the laser light incoherent (Approach 1), and otherinvolves reducing the perceived speckle (Approach 2).

Approach 1 is a method of canceling the coherence of laser light toconvert to incoherent light. The broadening of wavelength width by meansof high-frequency superimposition, the multiplexing of laser lighthaving a delay that is greater than the coherence length, or theoverlapping of orthogonal polarized light all pertain to Approach 1.Essentially, Approach 1 is a method of altering the characteristics oflight to control the generation of speckle.

In contrast, Approach 2 is a method for reducing apparent speckle byrepeatedly superimposing the (integral) speckle pattern in an image attime intervals (<40 msec) that are indistinguishable to the human eye toequalize speckle noise. Methods of vibrating the screen or opticscomponents belong to Approach 2. Methods that belong to Approach 2 donot alter the characteristics of light, and speckle is thereforegenerated. Approach 2 is a method that takes advantage of an illusion inthe human brain to make speckle imperceptible to the eye.

Of the methods that pertain to Approach 2 (reducing perceptiblespeckle), the method of reducing speckle noise by vibrating opticscomponents is taken up in the present specification.

FIG. 2 is a perspective view showing a first technique for reducingspeckle noise. FIG. 2( a) shows an example of the first technique, andFIG. 2( b) shows another example. The details of the first technique aredisclosed in JP-A-H11-064789. In the example shown in FIG. 2( a),optical integrator 17 a composed of two fly-eye lenses 13 c and 13 drotates around an optical axis. When the optics rotate, the specklepattern moves temporally and spatially in the optics, the speckle thatis image-formed on the retina is integrated, and the apparent specklenoise is reduced. In the example shown in FIG. 2( b), on the other hand,a similar effect is obtained by the rotation of rod-type opticalintegrator 19 a (a transparent medium such as glass having a rectangularcross-section) around the optical axis.

FIG. 3 is a block diagram showing a second technique for reducingspeckle noise. FIG. 3( a) shows an example of the second technique, andFIG. 3( b) shows another example. The details of the second techniqueare disclosed in JP-A-H07-297111. In the example shown in FIG. 3( a),diffusion plate 16 b that is caused to rotate by motor 20 is arrangedmidway on an optical path. When diffuser 16 b rotates, the scatteringstate on the optical path changes and the speckle pattern vibratestemporally and spatially, whereby the speckle that is formed as an imageon the retina is integrated and the apparent speckle noise is reduced.In the example shown in FIG. 3( b), diffuser 16 c that is arrangedmidway on an optical path is caused to vibrate by transducer 23. Whendiffusion plate 16 c vibrates, the apparent speckle noise is reduced dueto the same principles as previously described.

FIG. 4 is a sectional view showing a third technique for reducingspeckle noise. FIG. 4( a) shows an example of the third technique, andFIG. 4( b) shows another example. The details of the third technique aredisclosed in JP-A-2003-098476. In the example shown in FIG. 4( a),diffuser 16 d is arranged between beam expanding optics 25 that includeexpanding lens (collimator lens 3 f) and collimator lens 3 g and beamshaping optics 27 that include two fly-eye lenses 13 e and 13 f andcondenser lenses 14 h and 14 i. Diffuser 16 d is caused to vibrate bymovement-inducing means 26 a. When diffuser 16 d vibrates, the specklepattern vibrates temporally and spatially, whereby the speckle that isformed as an image on the retina is integrated and the apparent specklenoise is reduced. In addition, in the example shown in FIG. 4( b),diffuser 16 e is also arranged between beam shaping optics 27 andspatial optical modulation element 5 f. Diffusers 16 d and 16 e arecaused to vibrate by movement-inducing means 26 a and 26 b.

FIG. 5 is a structural diagram showing a fourth technique for reducingspeckle noise. FIG. 5( a) shows one example of the fourth technique andFIG. 5( b) shows another example. The details of the fourth techniqueare disclosed in WO2005/008330. In the example shown in FIG. 5( a),diffuser 16 f arranged midway in an optical path is connected todiffuser vibration section 28 a. Diffuser vibration section 28 a causesdiffuser 16 f to vibrate at a vibration speed V. Vibration speed V isset to satisfy the relation V>d×30 where d is the particle size ofdiffuser 16 f. WO2005/008330 discloses control of the diffusion angle ofa diffuser based on the relation between the numerical aperture of theillumination optics and the F-number of the projection lens to suppressthe optical loss of laser light caused by a diffuser. In the exampleshown in FIG. 5( b), rod-type optical integrators 19 b are used in placeof two fly-eye lenses 13 g and 13 h.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

A large-scale rotation mechanism is necessary for causing an opticalintegrator to rotate around its optical axis to integrate speckle, andthis requirement leads to an increase in costs and cubic volume ofpackaging, and further, to an increase in power consumption. Luminousflux irradiated into an optical modulation element passes through anoptical integrator that is revolving, and this motion introduces thepotential for shifting of the optical axis of the luminous flux from apredetermined position.

To supplement the explanation regarding power consumption, the use of alaser that is employed in place of a lamp in a projector eliminates theneed for separation optics and further reduces the size of the lightsource, thereby enabling miniaturization of the optics engine. Inaddition, power consumption of the laser light source is lower than alamp light source, whereby the realization of a battery-driven portableprojector can be anticipated. Realizing a portable projectornecessitates the reduction of power consumption of the overallapparatus, and an increase in power consumption of the mechanism forreducing speckle noise is therefore to be avoided.

A large-scale rotation mechanism or vibration mechanism is necessary forcausing a diffuser arranged midway in an optical path to rotate orvibrate, and such mechanisms therefore lead to an increase in costs oran increase in package volume. In addition, issues are also raisedsimilar to those described above relating to the increase of powerconsumption. Problems also arise relating to optical loss.

The optical transmittance of a diffuser is typically lower than that ofa lens (on the order of 80-90%). In addition, because the spacing of ahologram pattern becomes denser as the diffusion angle increases, theintegration effect of speckle that is caused by vibration increases butthe optical transmittance decreases. Accordingly, using a diffuserhaving a large diffusion angle to augment the speckle reduction effectresults in an increase of optical loss and a reduction of brightness. Inthe example shown in FIG. 4( b) in particular, two diffusers arearranged on an optical path. Such a case results in added specklereduction effect but also results in added optical loss, whereby theproblem of decreased luminance becomes severe.

Regarding the operating conditions of diffusers (frequency andamplitude), the following conditions can be considered.

Flickering of an image within a short time interval (<40 msec) isintegrated (averaged) in the human brain that processes the image and istherefore not perceived. Accordingly, if speckle images that havegreater movement than the average size of a speckle pattern aresuperimposed a plurality of times in a short time interval (<40 msec),the speckle pattern is integrated and becomes imperceptible.

Accordingly, when a diffuser is caused to vibrate to reduce the apparentspeckle, the vibration frequency of the diffuser must be set such thatthe movement speed of the speckle pattern that is formed as an image onthe retina exceeds the limit of human perception. In addition, thevibration amplitude of the diffuser must be set such that the amount ofdisplacement of the speckle pattern that is formed as an image on theretina exceeds the average size of the speckle pattern.

The diffuser vibration conditions that are required for reducing specklenoise are as shown below, where F is the vibration frequency of thediffuser, ±A is the vibration amplitude, ±a is the amount ofdisplacement of the speckle pattern on the retina, T is the time limitof perceiving flicker of an image, n is the number of superimpositionsof the speckle pattern (in the brain), and δ is the average size of thespeckle pattern.

$\begin{matrix}{{\left( {2{a/\delta}} \right) \times F} > {\left( {1/T} \right) \times n}} & \; \\{\therefore{F > {\left( {n \times \delta} \right)/\left( {2a \times T} \right)}}} & (1) \\{{{where}\mspace{14mu} a} > {\delta/2}} & (2)\end{matrix}$

When the diffusion plane moves, however, the speckle pattern also movesin accompaniment, but the amount of displacement (A) of a rough surface(diffusion plane) and the amount of movement of speckle (a) are in aproportional relation that is strongly dependent on the optics. If thisproportionality constant is set to k, the following equation applies:

a=k×A  (3)

Accordingly, expression (1) and expression (2) can be rewritten asfollows:

F>(n×δ)/(2k×A×T)  (1)′

A>δ/2k  (2)′

Because the average size δ of a speckle pattern is proportional to theproduct of the light source wavelength and the F-number of the pupil,the average size δ of a speckle pattern can be found by the followingequation, where λ is the wavelength of laser light, f is the focallength of an eyeball, and D is the diameter of the pupil:

δ=1.22λ×f/D  (4)

As can be understood from expressions (1)′ and (2)′, causing a diffuserto vibrate to average a speckle pattern requires that the vibrationfrequency (F) and vibration amplitude (A) of the diffuser be madegreater than certain threshold values. To the extent that thesethreshold values are surpassed and vibration frequency (F) and vibrationamplitude (A) increase, the number of superimpositions (n) of speckleincreases and the effect of reducing speckle noise is raised (however,this effect becomes asymptotic at a certain level). Accordingly, when adiffuser that is arranged on an optical path is vibrated to average aspeckle pattern, the diffuser must be vibrated at high frequency andgreat amplitude. However, causing a diffuser to vibrate at highfrequency and great amplitude tends to bring about an increase in powerconsumption and noise.

It is an object of the present invention to reduce speckle noise withoutbringing about an increase in power consumption or noise.

The object, features, and advantages of the above-described presentinvention other than described hereinabove will become obvious byreference to the following description and accompanying drawings showingexamples of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a schematic representation of aprojector that uses a laser light source;

FIG. 2 is a schematic view showing a first technique relating to specklereduction;

FIG. 3 is a schematic view showing a second technique relating tospeckle reduction;

FIG. 4 is a schematic view showing a third technique relating to specklereduction;

FIG. 5 is a schematic view showing a fourth technique relating tospeckle reduction;

FIG. 6( a) is a schematic perspective view, (b) is a schematic planview, and (c) is a schematic side view showing the first exemplaryembodiment of the present invention;

FIG. 7( a) is a schematic perspective view, (b) is a schematic planview, and (c) is a schematic side view showing the second exemplaryembodiment of the present invention;

FIG. 8( a) is a schematic perspective view, (b) is a schematic planview, and (c) is a schematic side view showing the third exemplaryembodiment of the present invention;

FIG. 9( a) is a perspective view of an analysis model of a diffuser inthe second exemplary embodiment, and (b) is a perspective view showingthe mode shape of the diffuser;

FIG. 10( a) is a perspective view of an analysis model of a diffuser inthe third exemplary embodiment, and (b) is a perspective view showingthe mode shape of the diffuser;

FIG. 11 is a block diagram showing the configuration of a water coolingsystem; and

FIG. 12 is a schematic perspective view showing the fourth exemplaryembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a projection-type image displayapparatus that uses a laser light source. The oscillation wavelength andoutput of the laser light source (semiconductor laser) are dependent ontemperature. Normally, the oscillation wavelength shifts toward a longerwavelength at the rate of approximately 0.3 nm/° C. as the temperatureof the semiconductor laser rises. On the other hand, the oscillationwavelength shifts toward a shorter wavelength at the rate ofapproximately 0.3 nm/° C. as the temperature drops. In addition, whenthe temperature of a semiconductor laser element becomes high, itsoutput drops. Accordingly, the temperature must be adjusted duringoperation to stabilize the oscillation wavelength and output of thesemiconductor laser. In other words, measures must be taken duringoperation such as cooling the semiconductor laser or lowering theambient temperature of the semiconductor laser.

The present invention features the reduction of speckle noise by using acooling mechanism that directly cools a laser light source or a coolingmechanism for lowering the ambient temperature of the laser lightsource.

Examples of exemplary embodiments of the projection-type image displayapparatus of the present invention are next described while referring tothe accompanying figures.

The optical modulation elements of the projection-type image displayapparatus of the present exemplary embodiment are liquid crystal panels,and the optical integrators are light tunnels. However, the opticalmodulation elements are not limited to liquid crystal panels, and theoptical integrators are not limited to light tunnels. For example, DMD(Digital Micro-mirror Devices) may be used for the optical modulationelements, and rod-type optical integrators or two fly-eye lenses may beused for the optical integrators. The basic configuration of theprojection-type image display apparatus has already been described andredundant description is therefore here omitted.

In the following explanation, only one channel of the R/G/B channels isdescribed in the interest of simplifying the explanation. The other twochannels that are not described have substantially the sameconfiguration.

FIG. 6 is a schematic view showing the first exemplary embodiment. Morespecifically, FIG. 6( a) is a schematic perspective view, FIG. 6( b) isa schematic plan view, and FIG. 6( c) is a schematic side view.

Projection-type image display apparatus 29 a shown in FIG. 6 includes:semiconductor laser 30 a that is the light source, diffuser 16 g, lighttunnel 4 d, condenser lenses 14 l and 14 m, liquid crystal panel 31, anddichroic prism 6. Polarization beam splitter (PBS) 8 f is arrangedbetween diffuser 16 g and semiconductor laser 30 a. Incidence-sidepolarizing plate 9 e is arranged on the light incidence side of liquidcrystal panel 31. Emission-side polarizing plate 10 e is arranged on thelight emission side of liquid crystal panel 31.

Polarization beam splitter (PBS) 8 f substantially decreases intransmittance when the angle of incidence of light diverges from theperpendicular direction, whereby polarization beam splitter (PBS) 8 f ispreferably arranged before the diffuser when the optics are made up fromthe combination of a diffuser and light tunnel. In other words,polarization beam splitter (PBS) 8 f is preferably arranged betweensemiconductor laser 30 a and diffuser 16 g.

Air-cooling fan 32 a for cooling semiconductor laser 30 a is arrangedclose to semiconductor laser 30 a. Air-cooling fan 32 a is connected todiffuser 16 g by means of a plate spring 33 a. A portion (fixed part 34a) of plate spring 33 a is secured to any point in the case (forexample, the holding stage of the semiconductor laser).

Laser light that is emitted from semiconductor laser 30 a is convertedto a specific linearly polarized light by polarization beam splitter(PBS) 8 f and irradiated into diffuser 16 g. Laser light that has passedthrough diffuser 16 g is irradiated into light tunnel 4 d. The beamdiameter of laser light that has passed through diffuser 16 g isgradually enlarged until it is irradiated into light tunnel 4 d. Thelaser light that is irradiated into light tunnel 4 d, in the process ofadvancing while being repeatedly reflected inside light tunnel 4 d, isshaped into luminous flux having a rectangular cross-section withuniform luminance distribution. The shaped laser light passes throughcondenser lenses 14 l and 14 m and is irradiated into optical modulationunit 35 that is made up from incidence-side polarizing plate 9 e, liquidcrystal panel 31 and emission-side polarizing plate 10 e. The laserlight that is irradiated into optical modulation unit 35 undergoesoptical modulation according to image signals. The laser light that hasundergone optical modulation is irradiated into dichroic prism 6 andsynthesized with the colored light of other channels not shown in thefigures. The synthesized laser light is then enlarged and projected ontoa screen by way of a projection lens (not shown).

Air-cooling fan 32 a both supplies cooling air current to semiconductorlaser 30 a and causes plate spring 33 a and diffuser 16 g to resonate ata solid propagation frequency.

For example, when the rotational speed of air-cooling fan 32 a is set to6500 rpm, the solid propagation frequency f_(D) (Hz) of air-cooling fan32 a is f_(D)=6500/60=108.33 (Hz).

If the mass of diffuser 16 g is m (kg) and the flexural rigidity K (N/m)of plate spring 33 a is K=m×(2λf_(D))², diffuser 16 g vibrates at aresponse amplitude of plate spring 33 a that corresponds to resonancefrequency f_(D).

In other words, the kinetic energy of the air-cooling fan that cools thesemiconductor laser can be used to cause the diffuser to vibrate at thesolid propagation frequency of the air-cooling fan. The amount ofdisplacement of the diffuser resulting from the vibration matches theresponse amplitude at the resonance point of the plate spring.Essentially, the diffuser can be caused to vibrate at high frequency andlarge amplitude.

Accordingly, the speckle pattern is effectively integrated and thespeckle noise is substantially reduced. Further, electric power is notconsumed to cause the diffuser to vibrate and audible noise is notincreased. In addition, because a simple configuration is employed thatinvolves simply connecting the air-cooling fan and diffuser by a platespring, a compact speckle-reducing construction can be realized at lowcost. These effects are the basic effects that are shared by each of thefollowing exemplary embodiments.

The second exemplary embodiment of the projection-type image displayapparatus of the present invention is next described. FIG. 7 is aschematic view showing the second exemplary embodiment. Morespecifically, FIG. 7( a) is a schematic perspective view, FIG. 7( b) isa schematic plan view, and FIG. 7( c) is a schematic side view.

In FIG. 7, the polarization beam splitter, the optical modulation unit(incidence-side polarizing plate, liquid crystal panel, emission-sidepolarizing plate), and the dichroic prism are omitted from the drawingsto emphasize the differences with the first exemplary embodiment.

Projection-type image display apparatus 29 b shown in FIG. 7 includestwo diffusers (first diffuser 36 a and second diffuser 37 a). Diffuser36 a and diffuser 37 a are connected to air-cooling fan 32 b for coolingsemiconductor laser 30 b by means of plate spring 33 b such that each ofdiffusers 36 a and 37 a can vibrate individually.

The rigidity of plate spring 33 b is designed such that first diffuser36 a and second diffuser 37 a vibrate at opposite phases to each otherat the solid propagation frequency of air-cooling fan 32 b. Plate spring33 b that receives the solid propagation vibration of air-cooling fan 32b during operation of air-cooling fan 32 b therefore resonates, andfirst diffuser 36 a and second diffuser 37 a that are arranged inparallel vibrate in opposite directions. The relative speed and relativedisplacement (resonance amplitude) of the two diffusers are thusdoubled. Accordingly, a sufficient speckle pattern integration effect isobtained even when using diffusers having a narrow angle of diffusionfor suppressing optical loss.

The third exemplary embodiment of the projection-type image displayapparatus of the present invention is next described. FIG. 8 is aschematic view showing the third exemplary embodiment. Morespecifically, FIG. 8( a) is a schematic perspective view, FIG. 8( b) isa schematic plan view, and FIG. 8( c) is a schematic side view.

In FIG. 8 as well, the polarization beam splitter, optical modulationunit (incidence-side polarizing plate, liquid crystal panel, andemission-side polarizing plate), and dichroic prism are omitted from thefigure.

In projection-type image display apparatus 29 c shown in FIG. 8, theoptical integrator is divided into first light tunnel 38 and secondlight tunnel 39. In addition, projection-type image display apparatus 29c includes two diffusers (first diffuser 36 b and second diffuser 37 b).First diffuser 36 b, first light tunnel 38, second diffuser 37 b, andsecond light tunnel 39 are arranged on the optical axis in that order.First diffuser 36 b and second diffuser 37 b are connected toair-cooling fan 32 c for cooling semiconductor laser 30 c by platespring 33 c such that each of diffusers 36 b and 37 b can vibrateindividually.

According to the construction of the present exemplary embodiment, thebeam diameter of laser light that is incident to the optical modulationunit (not shown) is enlarged two times and the luminance distribution isequalized two times, whereby luminous flux having an extremely limitedluminance irregularity and an ideal cross-section shape is obtained.

Plate spring 33 c is designed to have a resonance mode such that firstdiffuser 36 b and second diffuser 37 b vibrate at mutually oppositephases at the solid propagation frequency of air-cooling fan 32 c.Accordingly, as in the second exemplary embodiment, the relative speedand relative displacement of first diffuser 36 b and second diffuser 37b are doubled and a sufficient speckle pattern integration effect isobtained.

The vibration mode of a diffuser in the second exemplary embodiment andthird exemplary embodiment is next described based on the results ofeigenvalue analysis by means of simulation.

FIG. 9( a) is a perspective view of an analysis model of diffusers 36 aand 37 a in the second exemplary embodiment. FIG. 9( b) shows the modeshapes of diffusers 36 a and 37 a at the solid propagation frequency ofair-cooling fan 32 b.

The analysis model shown in FIG. 9( a) was modeled by picking out onlythe drive unit of diffusers 36 a and 37 a in the second exemplaryembodiment. First diffuser 36 a and second diffuser 37 a are connectedto air-cooling fan 32 b by way of plate spring 33 b. Calculation iscarried out on the assumption that fixed part 34 b of plate spring 33 bis completely secured. The mass of each of first diffuser 36 a andsecond diffuser 37 a is 1 g, and plate spring 33 b is fabricated fromstainless steel 0.5 mm thick.

In the above-described simulation by means of an analysis model, avibration mode is obtained in which, as shown in FIG. 9( b), firstdiffuser 36 a and second diffuser 37 a vibrate at mutually oppositephases in a vertical direction (a direction orthogonal to the opticalaxis) at 108.5 kHz that matches the solid propagation frequency ofair-cooling fan 32 b.

FIG. 10( a) is a perspective view of the analysis model of diffusers 36b and 37 b in the third exemplary embodiment. FIG. 10( b) shows the modeshape of diffusers 36 b and 37 b at the solid propagation frequency ofair-cooling fan 32 c.

The analysis model shown in FIG. 10( a) was modeled by picking out onlythe drive unit of diffusers 36 b and 37 b in the third exemplaryembodiment. First diffuser 36 b and second diffuser 37 b are connectedto air-cooling fan 32 c by way of plate spring 33 c. Calculation iscarried out on the assumption that fixed part 34 c of plate spring 33 cis completely secured. The mass of each of first diffuser 36 b andsecond diffuser 37 b is 1 g, and plate spring 33 c is fabricated ofstainless steel that is 0.5 mm thick.

In the simulation of the above-described analysis model, a vibrationmode is obtained in which, as shown in FIG. 10( b), first diffuser 36 band second diffuser 37 b vibrate at mutually opposite phases in avertical direction (a direction orthogonal to the optical axis) at 108.8kHz that matches the solid propagation frequency of air-cooling fan 32c.

By causing two diffusers to vibrate at opposite phases in this way, thespeckle pattern of laser light that passes through the two diffusers iseffectively averaged, and speckle noise is substantially reduced.

The shape of the plate spring shown in the figure is only an example.The shape of the plate spring is not limited to any particular shape aslong as the shape obtains a desired vibration mode at a predeterminedfrequency (the solid propagation frequency of the fan).

The fourth exemplary embodiment of the projection-type image displayapparatus of the present invention is next described. FIG. 11 is a blockdiagram of a water cooling system used in the present exemplaryembodiment.

Water cooling system 40 a shown in FIG. 11 includes heating jacket 41 a,radiator 42 a, circulation pump 43 a, and reserve tank 44 a. Heatingjacket 41 a is thermally connected to heating element 45 and absorbs theheat produced by heating element 45. The heat absorbed by heating jacket41 a is conveyed to radiator 42 a by way of a coolant liquid that flowsinside heating jacket 41 a. In radiator 42 a, the coolant liquid iscooled by the thermal exchange (radiation) between the coolant liquidand the outside air. Natural air cooling or forced air cooling is usedin cooling of the coolant liquid in radiator 42 a. The cooled coolantliquid is conveyed to heating jacket 41 a by way of reserve tank 44 a bycirculation pump 43 a. Reserve tank 44 a compensates for loss of coolantliquid due to volatilization and thus maintains the amount of coolantliquid in the system.

The above-described water cooling system features quiet operation andhigher cooling performance than an air cooling system and is thereforesuited to quiet cooling of a heating element that gives off a largevolume of heat. The above-described water cooling system can thereforebe adopted as the cooling means of a semiconductor laser when ahigh-output semiconductor laser is used in a high-luminance projector.

FIG. 12 is a schematic perspective view of the projection-type imagedisplay apparatus of the fourth exemplary embodiment. In FIG. 12 aswell, polarization beam splitter, optical modulation unit(incidence-side polarizing plate, liquid crystal panel, emission-sidepolarizing plate), and dichroic prism are omitted from the figure.

In projection-type image display apparatus 29 d shown in FIG. 12, firstdiffuser 36 c and second diffuser 37 c are connected to circulation pump43 b of water cooling system 40 b by way of plate spring 33 d. Platespring 33 d is designed such that first diffuser 36 c and seconddiffuser 37 c vibrate at mutually opposite phases at the solidpropagation frequency of circulation pump 43 b. Thus, when water coolingsystem 40 b is applied to the cooling of semiconductor laser 30 d, thesame effect as the above-described second exemplary embodiment isobtained.

Circulation pump 43 b in FIG. 12 corresponds to circulation pump 43 a inFIG. 11. Heating jacket 41 b similarly corresponds to heating jacket 41a. Radiator 42 b corresponds to radiator 42 a. Reserve tank 44 bcorresponds to reserve tank 44 a. Semiconductor laser 30 b correspondsto heating element 45.

In FIG. 12, an example is shown in which the air cooling system of thesecond exemplary embodiment is replaced by a water cooling system, butthe air cooling system of the first exemplary embodiment or the thirdexemplary embodiment can also be replaced by a water cooling system.

A low-output semiconductor laser may be used as a light source in alow-luminance projector. In such cases, a cooling means (air coolingfan) dedicated to the semiconductor laser may not be provided.

In the case described hereinabove, the diffusers installed in each ofchannels R/G/B are connected as a group to an exhaust fan by way of aplate spring. The exhaust fan is a fan for discharging air inside thecase to the outside in order to discharge heat. In this construction,each of the diffusers resonates at the solid propagation frequency ofthe exhaust fan. However, the distance between the exhaust fan and thediffuser of each channel differs. With respect to the plate spring, eachof the following differ in length: a first part that connects theexhaust fan and the R-channel diffuser, a second part that connects theexhaust fan and the G-channel diffuser, and a third part that connectsthe exhaust fan and the B-channel diffuser. As a result, the platespring must be designed such that the first to third parts have a commonresonance frequency.

Alternatively, only the G-channel diffuser that has the most influenceupon speckle contrast (to be described) of white light is connected tothe exhaust fan and caused to vibrate. In the case of a low-luminanceprojector, a certain visual effect is obtained even when only thespeckle noise of green laser light is eliminated.

Finally, the speckle reduction effect realized by the present inventionis next described based on the results of actual measurement. Generally,speckle contrast is known as an index for quantitative evaluation of theintensity of speckle noise. Speckle contrast (η) is expressed by theratio of the standard deviation (σ₁) of speckle pattern intensity andaverage intensity (I_(AVE)) of the speckle pattern, as shown in thefollowing equation:

η=σ₁ /I _(AVE)

Accordingly, when N is the number of data items (number of speckle imagepixels) and I(n) is the luminance of the n^(th) data item (pixel), thestandard deviation (σ₁) of speckle pattern and the average intensity(I_(AVE)) are as shown by the following equation:

{Equation 1}

A speckle image (first speckle image) of the state in which the aircooling fan is stopped in the second exemplary embodiment and a speckleimage (second speckle image) of the state in which the air-cooling fanis in operation are photographed and a luminance histogram of each ofthe speckle images is found. A speckle image is an image (surface lightsource image) shown on a screen that is arranged in front of theemission port of the light tunnel shown in FIG. 7.

A mottled speckle pattern was revealed in the first speckle image. As aresult, the standard deviation (σ₁) of the luminance histogram increasedin size and speckle contrast (η) also exhibited a large value (η=11.8%).

On the other hand, in the second speckle image, the speckle pattern wasconsiderably reduced compared to the first speckle image. As a result,the standard deviation (σ₁) of the luminance histogram was smaller andspeckle contrast (η) also exhibited a small value (η=4.9%). In otherwords, speckle contrast was reduced more than 58%.

Thus, according to the present invention, a speckle reductionconstruction that obtains a sufficient speckle reduction effect isrealized at low cost. The speckle reduction construction according tothe present invention further features smaller size, lower noise, andlower power consumption.

1. A projection-type image display apparatus that enlarges and displaysan image, comprising: a case; a laser light source; a cooling mechanismthat includes a component that produces vibration during operation; adiffuser that diffuses light emitted from said laser light source;optics that shape light that is diffused by said diffuser to lighthaving a uniform luminance distribution and rectangular cross-section;and an optical modulation element that modulates light shaped by saidoptics; wherein said component included in said cooling mechanism andsaid diffuser are connected by way of a vibration transmitting member.2. The projection-type image display apparatus as set forth in claim 1,wherein the solid propagation frequency of said component included insaid cooling mechanism matches the characteristic frequency of saidvibration transmitting member.
 3. The projection-type image displayapparatus as set forth in claim 1, further comprising a plurality ofsaid diffusers, wherein each of these diffusers is connected to saidcomponent included in said cooling mechanism by way of said vibrationtransmitting member.
 4. The projection-type image display apparatus asset forth in claim 3, wherein the solid propagation frequency of saidcomponent included in said cooling mechanism matches the characteristicfrequency of said vibration transmitting member.
 5. The projection-typeimage display apparatus as set forth in claim 3, wherein said opticsinclude a plurality of optical elements, and the plurality of opticalelements and said plurality of diffusers are arranged alternately. 6.The projection-type image display apparatus as set forth in claim 3,wherein the vibration mode of said vibration transmitting member at thesolid propagation frequency of said component included in said coolingmechanism has a mode shape that causes said plurality of diffusers tovibrate at mutually opposite phases.
 7. The projection-type imagedisplay apparatus as set forth in claim 1, wherein said vibrationtransmitting member comprises a plate spring.
 8. The projection-typeimage display apparatus as set forth in claim 1, wherein said componentincluded in said cooling mechanism comprises a fan that supplies coolingair to said laser light source.
 9. The projection-type image displayapparatus as set forth in claim 1, wherein said component included insaid cooling mechanism comprises a fan that discharges air inside saidcase to the outside.
 10. The projection-type image display apparatus asset forth in claim 1, wherein said component included in said coolingmechanism comprises a pump for circulating a coolant liquid.